GERMAN METHOD FOR SLAGS RICH IN SILICIC ACID ' 2OO, 



be exercised that the reagents employed in the preparation of the 

 molybdic solutions be absolutely pure. 



b. Molybdic solutions before using should be tested for purity 

 by means of a solution of disodium phosphate. 



c. The mixture of the citric acid extract and the molybdic solu- 

 tion is to be taken from the water bath when the prescribed 

 temperature has been reached. If the time of digestion be mark- 

 edly extended a contamination of the precipitate with silicic acid 

 arises, especially when the citric acid extract is not in a fresh 

 condition, but only after from six to 12 hours standing is 

 treated with the molybdic solution. 



d. A contamination of the molybdic precipitate with the silicic 

 acid is recognized by the following: Slow solution of the pre- 

 cipitate in ammonia and the production of a solution not com- 

 pletely clear or becoming only slowly so. Under these condi- 

 tions the process already advised, namely, a reprecipitation of 

 the magnesia precipitate, is to be applied. 



183. The Official German Method for Slags Rich in Silicic Acid. 

 When the slags are very rich in soluble silicic acid the process 

 of analysis is conducted as follows: 55 



The sample is to be tested for silicic acid by the method of Kell- 

 ner, which consists in boiling for one minute 50 cubic centime- 

 ters of the citric acid extract with 50 cubic centimeters of ammo- 

 niacal citrate solution and allowing to stand for a few minutes. 56 

 If there is sufficient silicic acid present to interfere with the direct 

 (Bottcher) precipitation of the phosphoric acid, a precipitate 

 is separated which is not entirely soluble in hydrochloric acid. 

 The ammoniacal citrate solution, employed above, contains in 10 

 liters, iioo grams of nitric acid, 4000 grams of 24 per cent, am- 

 monia, and water to the mark. 



The presence of a disturbing amount of silicic acid having 

 been thus determined, it is separated in the following manner : 

 To loo cubic centimeters of the citric acid extract of the slag 

 3re added 7.5 cubic centimeters of hydrochloric acid of 1.12 

 specific gravity or five cubic centimeters of fuming hydrochloric 

 55 Die landwirtschaftlichen Versuchs-Stationen, 1904, 60 : 374; 1905, 



1 =351- 



58 Chemiker-Zeitung, 1902, 26 : 1151. 



